1. Field of the Invention
This invention relates to improvements in reduction of viscosity of heavy residua, and in particular to an improved hydrovisbreaking process and apparatus.
2. Description of Related Art
Heavy residua such as atmospheric or vacuum residues generally require varying degrees of conversion to increase their value and usability, including the reduction of viscosity to facilitate subsequent refining into light distillates products such as gasoline, naphtha, diesel and fuel oil. One approach to reduce the viscosity of heavy residua is to blend heavy residua with lighter oil, known as cutter stocks, to produce liquid hydrocarbon mixtures of acceptable viscosity. However, this has the disadvantage of consuming valuable, previously fractioned liquid hydrocarbon mixtures.
Other processes for conversion of heavy residua into light distillates and reduction in the viscosity include catalytic processes such as fluid catalytic cracking, hydrocracking, and thermal cracking processes such as visbreaking or coking. These processes increase the product yield and reduce the requirement for valuable cutter stock as compared to blending alone.
Thermal cracking processes are well established and exist worldwide. In these processes, heavy gas oils or vacuum residues are thermally cracked in reactors which operate at relatively high temperatures (e.g., about 425° C. to about 540° C.) and low pressures (e.g., about 0.3 bars to about 15 bars) to crack large hydrocarbon molecules into smaller, more valuable compounds.
Visbreaking processes reduce the viscosity of the heavy residua and increase the distillate yield in the overall refining operation by production of gas oil feeds for catalytic cracking. To achieve these goals, a visbreaking reactor must be operated at sufficiently severe conditions to generate sufficient quantities of the lighter products.
There are two types of visbreaking technologies that are commercially available: ‘coil’ or ‘furnace’ type processes and ‘soaker’ processes. In coil processes, conversion is achieved by high temperature cracking for a predetermined, relatively short period of time in the heater. In soaker processes, which are low temperature/high residence time processes, the majority of conversion occurs in a reaction vessel or a soaker drum, where a two-phase effluent is maintained at a comparatively lower temperature for a longer period of time.
Visbreaking processes convert a limited amount of heavy oil to lower viscosity light oil. However, the asphaltene content of heavy oil feeds severely restricts the degree of visbreaking conversion, likely due to the tendency of the asphaltenes to condense into heavier materials such as coke, thus causing instability in the resulting fuel oil.
Certain visbreaking processes which incorporate hydrogen gas in the thermal process to convert heavy oils, known as hydrovisbreaking, not only thermally crack the molecules into less viscous compounds, but also serve to hydrogenate them. The temperature and pressure of hydrogenation increase with increasing average molecular weight of the feedstock to be converted.
In conventional hydrovisbreaking processes, liquid-gas two-phase unit operations are required, thus necessitating relatively large reaction vessels and gas recycle system. This adds substantial capital investment and processing costs to the hydrovisbreaking operation, thereby minimizing fundamental advantages of hydrovisbreaking, i.e., lowering viscosity while reducing the quantity of cutter stock required.
Therefore, a need exists for improved processes for converting heavy residua.